MTR2000 SINAD and HP 8920

[gmrs122]

I have a MTR2000 and wanted to check the 12dB SINAD using a recently purchased HP 8920A. My question is very basic but I am finding conflicting information in the documentation. I believe I understand how the measurement works but I am uncertain how to correctly connect to make the measurement using the service monitor. My understanding is that the SINAD measurement looks at the AF audio out of the receiver (or repeater in my case) basically at the speaker output (or dummy 8ohm load). I have the Motorola external amplified speaker and cable so I can access the AF/speaker output of the MTR2000.

My question is how to connect the repeater speaker/AF out to the 8920? The 8920 Applications Handbook shows that for the SINAD measurement the receiver audio out connects to the 8920 audio out. I don't get that (why the 8920 audio out port) . Elsewhere it indicates that AF/AC measurements use the 8920 audio in connector. That makes more sense to me. I would think that the MTR2000 speaker/audio out should go to the 8920 audio in, not the audio out connector. But the 8920 applications handbook clearly show the receiver speaker connected to the 8920 audio out BNC.

So for people that have done this using a 892x, what is the correct 8920 audio port to use?

Thanks

[Bill_G]

I'm not familiar with the 8920, but you are correct. In general, the receive audio from the radio connects to the audio in / sinad in port of the service monitor generating the test signal. The sinad section is comparing the recovered 1000hz tone from the radio under test to the tone it sent through the generator so it can determine both the signal to noise ratio and the audio distortion figure. In general, that recovered audio is picked off the speaker leads of the radio under test. However, the MTR series lacks both a speaker, and a high level audio driver section. You have to modify the cable of the amplified spkr. Use this linked pdf diagram as your guide for making a sinad test cable.

[RFI-EMI-GUY]

I think I also recall seeing a connection mistake in the line drawings for one of the SINAD procedures. There is a Yahoo bulletin board for the HP8924 and HP8920. If you sign up there, you can download a live video I posted there which shows hthe 8920 being used for actual testing. It is a bit grainy due to being converted from VHS, but you can see what ports are being used, etc.

[jry]

The 8920 /21 /24 have two audio inputs. One high impedance and one low .

For the MTR you really do not care about the external speaker so I would connect the audio from the RJ45 connection to the 8920.

You could add a BNC cable to the amplified speaker at the input.

[MotoFAN]

I have recently did it with my Kenwood radio and HP 8922A/M. I used Lo port in and run audio analyzer (Af Anl In: Audio In, Audio In Lo: Float, Ext Load R: 4.0). Put speaker wires out and connected them to 4-Ohm resistor (5W minimum) via two of 220 uF capacitors (per Kenwood SM). Run RF generator and adjusted audio volume on radio to get 1VAC at 4-Ohm load.

After that, I aligned receiver accordingly to SM.

About RF generator. Of course, you must use 1000 Hz audio tone, because HP SINAD test designed only for 1000 Hz with +-5Hz drift as maximum. In my case with Kenwood RF generator was set to 1000 Hz FM modulation with 3 kHz deviation (something deviation must be set to 2.5, 4 or 5 kHz depending on your channel bandwidth, filter set, application, etc.).

However, I am not guru and worked with SINAD for a first time! After reading HP user guide and Kenwood SM I am not sure that:

1. Low power port is the best option. Why not High? I don’t know!
2. That 4.0 Ohm that I set is impedance of my speaker equivalent (or something else?)
3. That I must use Float option instead of Gnd

Because I did not find any valuable info about these questions in both SM and HP UG! Therefore, I will be glad to receive some comments on it!

P.S.
Took some pictures for future use:



There is no radio connected on this photo. But if connected it shows floating SINAD level from 10 to 14 dB at -120.0dBm.

[jry]

Normally you should be connecting an audio signal referenced to ground to the Hi input.

The Hi and Lo audio inputs are used for a differential audio output that some radios have on their speakers.

That provides a more accurate way of final test on the radio including the full audio PA.

The load resistance value is used to calculate the audio power out IIRC.

In the context of aligning the RF for peak performance it may not make much difference which one you use and what you put in for the load resistance..


The diagram in the 8920 application manual is in-correct and shows connecting the speaker to the audio output ( Tone ) instead of the audio Hi and Lo across the speaker.